In the de-icing and anti-icing of aircraft wings, horizontal stabilizers, or the like, a plurality of techniques has been utilized in the prior art. For example, pneumatic boots of rubber or other flexible material are provided to overlay the leading edges and are periodically provided with pulses of air or other aeroform fluid to de-ice by causing the periodic expansion and contraction of the surface in a manner to crack and remove any ice which might otherwise be formed thereon. Failure to remove the ice would cause a lack of manuverability of the aircraft. In an alternate approach, particularly in jet aircraft, the engine-bleed-exhaust may directly heat the wings of the aircraft including the leading edge thereof to anti-ice by precluding ice formation in the first instance. A third technique, and that employed in accordance with the instant invention, includes the electrical energizing of resistance elements formed in the leading edges of aircrafts. Electrical energy is used to heat the resistance elements either to raise the leading edge to a temperature to melt the wing-to-ice bond for removing any ice which may have formed thereon. It can also be used to maintain the leading edge at a temperature range to prevent ice from forming in an anti-icer mode. In this third electrical technique, as in any similar electrical technique, electrical power is needed to achieve the de-icing or anti-icing function.
Typical arrangements for energizing electrical resistance pads for anti-icing or de-icing in aircrafts are disclosed in several prior art patents including U.S. Pat. No. 3,183,975 to Keen; U.S. Pat. No. 4,010,412 to Forman; and U.S. Pat. Nos. 4,036,457 and 3,420,476, the last two patents being issued to Volkner, et al. In each of these prior art disclosures, power is periodically provided to a plurality of resistance elements on the leading edges of aircraft wings or horizontal stabilizers for sequentially heating said elements to maintain the surface in an ice removing mode. In each of these devices, however, as in all the prior art, it is not clear that sufficiently convenient means are provided for detecting whether or not the proper current is flowing in a commercially acceptable manner to ensure proper safety of the aircraft and its occupants.
In the prior art, there is no technique found for monitoring the current to a load which does not include high cost, weight or other disadvantages. Typical electrical current monitors presently known which, theoretically, could be used in the de-icer or anti-icer applications consist basically of devices for directly measuring current by any one of various means: (a) the Hall effect, (b) measuring of the voltage drop across a known resistor in series with the load and using Ohms law to determine the current, and (c) using the electromagnetic field in a coil energized by either AC or DC current, as for example, current transformers, galvonometer movement or sensitive current relays, whether AC or DC.
In this regard, various prior art devices are known for measuring current in other than direct measurement and include the patents to Boros et al; Born et al; Harris, Jr. and Durbeck. Boros et al, U.S. Pat. No. 4,315,316 discloses a digital current monitoring system for estimating the current output of a switching regulator by monitoring the voltage on the regulator. The output of a voltage controlled oscillator is numerically compared to a reference value by means of a microprocessor. A state variable equation is involved to determine the current.
The patent to Born et al, U.S. Pat. No. 4,074,175 teaches the use of a circuit for generating a signal proportional to the average current of an inductive load. Current is directly measured in a free path and also in a load path. The two signals are then combined to provide an average current signal.
In the Boros patent, the voltage is monitored while current is inferred without reference to the impedance of the circuit. In Born et al, a loaded and unloaded current are directly measured and these measurements are used to calculate the average load current.
The patent to Harris, Jr., U.S. Pat. No. 3,445,763 shows an impedance measuring digital circuit including a differential amplifier.
Lastly, the patent to Durbeck, U.S. Pat. No. 3,569,785 shows a circuit which measures voltage, then current and calculates impedance.
These disclosures, like the totality of the prior art, require the knowledge of the parameters of the load or source or the like or, in the alternative, use known current detection techniques in achieving their objectives.